BIOL 1005 Introduction to Biology Lecture 4 PDF

BIOL 1005 Introduction to Biology Lecture 4 Allen Cheung 22 Jan 2025 Carbohydrates Sugars Contained in rice, noodle, bread, cakes, snacks, etc. Our body and cells all require carbohydrates to function E.g. Brain requires glucose (110-145g per day) D...

BIOL 1005 Introduction to Biology Lecture 4 Allen Cheung 22 Jan 2025 Carbohydrates Sugars Contained in rice, noodle, bread, cakes, snacks, etc. Our body and cells all require carbohydrates to function E.g. Brain requires glucose (110-145g per day) Daily intake recommended as 225-325g per day What are carbohydrates? Carbohydrates, along with lipids, proteins, nucleic acids, and other compounds are known as biomolecules Compounds of tremendous biological importance - provide energy through oxidation - supply carbon for the synthesis of cell components - serve as a form of stored chemical energy - form part of the structures of some cells and tissues  Carbohydrates General molecular formula is Cn(H2O)n (a hydrate of carbon) 5 carbon carbohydrate will be C5H10O5 Monosaccharides (simple sugars) are the basic building blocks of carbohydrates A single continuous chain of carbons Classification of carbohydrates Monosaccharides Disaccharides Oligosaccharides Polysaccharides The term “carbohydrate” comes from the fact that when you heat sugars, you get carbon and water.  Saccharides Derived from the Greek word “saccharo” = sugar Scientists use saccharides to describe sugars Mono = 1 single polyhydroxy aldehyde or ketone (e.g. glucose, fructose) Di = 2 simple molecules bound together (e.g. sucrose) Oligo = 3-10 (e.g. raffinose) Poly = 10 or more long chains of monosaccharides in straight or branches (e.g. cellulose, starch) Monosaccharides Simplest form of carbohydrates Further classified according to: # Carbon atoms in backbone Functioning group (aldehyde or ketone)  General structure of carbohydrates Molecular formula is Cn(H2O)n (a hydrate of carbon) 5 carbon carbohydrate will be C5H10O5 Monosaccharides (simple sugars) are the basic building blocks of carbohydrates A single continuous chain of carbons Physical properties of monosaccharides Most monosaccharides have a sweet taste (fructose sweetest, 73% more than sucrose) Solids at room temperature Extremely soluble in water - despite their high MW, the presence of large number of OH groups make the monosaccharides much more water soluble than most molecules of similar MW - glucose can dissolve in minute amounts of water to make a syrup (1g /1ml water)  Table 7.2 The relative sweetness of sugars (sucrose = 1.00) Sugar Relative Type Sweetness Lactose 0.16 Disaccharide Galactose 0.22 Monosaccharide Maltose 0.32 Disaccharide Xylose 0.40 Monosaccharide Glucose 0.74 Monosaccharide Sucrose 1.00 Disaccharide Invert sugar 1.30 Mixture of glucose and fructose Fructose 1.73 Monosaccharide 28 Important monosaccharides 1 Important Monosaccharides CH2OH OH CH2OH OH O O OH OH OH β-D-ribose CH2OH β-D-deoxyribose Forms the sugar backbone of Forms the sugar backbone of ribonucleic acid (RNA) OH O OH deoxyribonucleic acid (DNA) OH OH β-D-galactose Incorporated with glucose into lactose (milk sugar) 39 Important monosaccharides 2 Important Monosaccharides CH2OH CH2OH OH O OH O HO OH CH2OH OH OH β-D-fructose OH Also known as levulose and fruit β-D-glucose sugar. Fructose is the sweetest of Also known as dextrose and blood the monosaccharides. It is present sugar; present in honey and fruits. in honey (1:1 ratio with glucose), Glucose is metabolized in the body fruits, and corn syrup. It is often for energy. Other sugars absorbed used to sweeten foods, since less into the body must be converted to fructose is needed to achieve the glucose by the liver. same degree of sweetness. Disaccharides  Disaccharides Two monosaccharides linked by a glycosidic bond forms a disaccharide Physiologically important disaccharides are: - Sucrose - Lactose - Maltose  Galactose Glucose lactose sucrose Found in milk of mammals Found in sugar cane and sugar beets maltose Major degradation product of starch Polysaccharides Oligosaccharides Contain 3-10 molecules of monosaccharide units e.g. Maltotriose (glucose + glucose + glucose) Polysaccharides Contain more than 10 molecules of monosaccharide units Homopolysaccharides and heteropolysaccharides Polysaccharides Most of the carbohydrates found in nature occur in the form of polysaccharides The main building blocks in polysaccharides is D-glucose When polysaccharides are composed of a single monosaccharide, they are termed homopolysaccharides When polysaccharides are composed of more than one type of monosaccharide are termed heteropolysaccharides  How is excess sugar (carbs) stored? In plants, glucose is stored in the form of polysaccharides = starch in their roots In animals, glucose is stored in the form of polysaccharide known as glycogen in our liver and muscle cells Mixture of 2 types of polysaccharides 1. Amylose 2. Amylopectin Highly branched, side chains ~30 glucose units, linked with ~200 - 20,000 glucose 1→6 glycosidic bond per 20- units in a chain that form a 30 units. Up to 2 million units. helix  Amylose and Amylopectin Amylose is a glucose polymer with (1,4) glycosidic bond Amylopectin is a glucose polymer with mainly (1,4) glycosidic bond and branches formed by (1,6) linkages Branches produce a compact structure and provide multiple chain ends at which enzymatic cleavage of the polymer can occur Glycogen is a polymer of -D- glucose identical to amylopectin, but branches are shorter ~10 glucose units≈ Originates from a pair of glycogenin molecules Easily converts to glucose to provide energy Stored in animal tissues Glucose not stored as glycogen or used immediately will be converted to fat  Glycogen Major form of stored carbohydrate in animals Homopolymer of glucose linked by (1,4) glycosidic bond Like amylopectin, it is also highly branched with a (1,6) branch linkage every 10 residues  Cellulose 1 Most abundant organic molecule on earth Gives trees and plants structure and strength Most animals cannot break the glucose linkage by normal means of hydrolysis → need special enzymes We need cellulose (fiber) to keep our digestive tracts clean and healthy  Cellulose 2 Major constituent of plant cell walls, long linear chains of glucose with (1,4) glycosidic bond  Summary - Carbohydrates Classification of carbohydrates Stereochemistry Mono-, Di-, Poly- saccharides Starch, Cellulose, Glycogen, Chitin